1. Field of the Invention
The present invention relates to a photographic element useful for holographic recording and for reproducing images using coherent rays emitted from lasers, and most especially to photographic emulsions of finely-divided silver halide particles spectrally sensitized by certain kinds of sensitizing dyes which have a high sensitivity in the red-sensitive range of 615-660 m.mu..
2. Description of the Prior Art
Sensitive elements used in holographic recording must have sufficient sensitivity to the laser beam used, i.e., they must have a high spectral sensitivity to the wavelength of the laser beam used.
Laser beams have an intrinsic wavelength which depends upon the kind of laser used; that is the substance which is the laser oscillation medium. The laser wavelength usually has a very high monochromatic characteristic.
Nowadays, the lasers most widely used include those using a ruby, a neon and helium gas, an argon gas, a krypton gas and a carbon dioxide gas as the laser oscillation medium. Among these, the laser beam obtained using a neon and helium gases as the laser oscillation medium is most widely used because of low cost and stable out-put. This type of laser is generally termed a neon-helium laser. In holography this neon-helium laser is now most preferably used, as the wavelength of the laser beam emitted from a neon-helium laser is 632.8 m.mu. in most cases.
It is necessary that a sensitive silver halide photographic element suitable for holographic recording using a neon-helium laser have a high spectral sensitivity to the 632.8 m.mu. wavelength of the neon-helium laser. Further, it is required that the sensitive element have a high resolving power because the space frequency of a holographic image to be recorded and reproduced usually exceeds 1000/mm. Accordingly, it is necessary that such a photographic emulsion have very low light scattering. Moreover, such a photographic emulsion should have an extremely low image granulation. In order to satisfy these conditions, it is necessary to use photographic emulsions containing extremely fine silver halide particles, namely those having an average particle size below 0.1 micron.
A problem is encountered in sensitizing such emulsions comprising extremely fine silver halide particles. Specifically, sensitizing dyes which afford an excellent spectral sensitization function to silver halide emulsions of a comparatively large particle size do not always afford an excellent spectral sensitization function to the silver halide emulsions of such small particle size.
Accordingly, for high resolution holographic recording and reproduction, it is necessary to discover sensitizing dyes which provide an excellent spectral sensitization function to silver halide photographic emulsions having a very small particle size.